Microencapsulated polymorphic agriculturally active material

ABSTRACT

Agriculturally active materials, such as pesticides, herbicides and the like and a process for microencapsulating said materials. The present invention includes trifluralin having a polymorphic form characterized by a yellow color and a melting point of approximately 41°-43° C. The present invention also includes a method of using the yellow polymorphic form of trifluralin as a herbicide. In addition, the present invention includes a method of microencapsulating low-melting agriculturally active materials, such as trifluralin. The microencapsulation process comprises the steps of heating a low-melting, water-immiscible, agriculturally active material to a molten state. The molten active material is then combined with an aqueous soluion of a water-soluble, film-forming polymer at a temperature sufficient to maintain the active material in its molten state. The active material is then dispersed or emulsified in the aqueous solution so that the active material has a primary particle size of between approximately 0.1 and 10 microns. The resulting dispersion or emulsion is then spray dried at a temperature between approximately 50° and 220° C., so as to microencapsulate the active material in the polymer. Compounds for promoting the rapid solidification of the active material are also disclosed.

FIELD OF THE INVENTION

The present invention relates generally to microencapsulatedagriculturally active materials, such as pesticides, herbicides and thelike, and to a process for producing said microencapsulated materials.More particularly, the present invention relates to a microencapsulatedpolymorphic form of trifluralin and other herbicides displaying two ormore polymorphic forms and to a process for making and using the same.The present invention also relates to a process for microencapsulatinglow-melting agriculturally active materials, to crystallizationinitiators and stabilizers for said agriculturally active materials andto a process for reducing nitrosamine impurities whilemicroencapsulating agriculturally active materials containingnitrosamines.

BACKGROUND OF THE INVENTION

Agriculturally active materials, such as pesticides, herbicides and thelike, are widely used throughout the agricultural industry. Forconvenience in packaging and handling, such agriculturally activematerials are typically produced in the form of a dry solid, such as apowder, which can be readily mixed with water. The aqueous solution ordispersion of the active material is then typically applied to an areato be treated by spraying.

In order to produce aqueous solutions or dispersions suitable forapplication by spraying, the agriculturally active material must be in aform which can be readily incorporated with water. The agriculturallyactive material by itself, however, is usually insoluble in water or hasan unacceptably low water solubility. Therefore, it is usually necessaryto treat the agriculturally active material in some manner to enhanceits combinability with water.

One such method of treating agriculturally active materials ismicroencapsulation. For example, U.S. Pat. No. 4,280,833 discloses aprocess for microencapsulating water-immiscible materials, such asherbicides, and, specifically, trifluralin. The microencapsulationprocess involves an aqueous phase containing an emulsifier and anorganic phase. The organic phase consists of a water-immisciblematerial, such as trifluralin, and polymethylene polyphenylisocyanate.The organic phase is added to the aqueous phase with agitation to form adispersion of small droplets of the organic phase within the aqueousphase. Thereafter, a polyfunctional amine is added to the dispersion.The polyfunctional amine reacts with the isocyanate to form a capsularpolyurea shell about the herbicide droplet. This type ofmicroencapsulation process is termed interfacial polycondensation. Otherpatents which involve microencapsulation by polycondensation processesinclude U.S. Pat. Nos. 4,360,376; 4,417,916; 4,563,212; 3,429,827;3,577,515; 3,959,464 and 4,640,709.

Another process used to microencapsulate active materials is spraydrying. U.S. Pat. No. 4,244,836 relates to a microencapsulation processusing spray drying. In that process, a liquid, water-insoluble phase isdispersed in an aqueous phase. The liquid, water-insoluble phase can bematerials, such as plant protecting agents. The aqueous phase is awater-soluble polyvinyl alcohol solution. The water-insoluble phase isdispersed in the aqueous phase using a stirrer or a homogenizationdevice so as to produce droplets of the water-insoluble phase of from 1to 50 microns in diameter within the aqueous phase. The dispersion inthen atomized into a stream of heated air (spray dried). The spraydrying dehydrates the aqueous dispersion and produces a dry powderymicrocapsular product. Other patents which utilize a spray dryingtechnique for microencapsulation include U.S. Pat. Nos. 4,286,020;4,353,962 and 4,690,786.

Active materials which can be microencapsulated include plant protectingagents, such as herbicides. Many conventional herbicides, particularlydinitroaniline-based herbicides, contain nitrosamine impurities. Sincenitrosamines are recognized carcinogens and since current governmentalregulations limit the amount of nitrosamines which herbicides cancontain, it is desirable to reduce the level of nitrosamines present inherbicides, particularly trifluralin. Various methods are known forreducing the level of nitrosamine impurities in herbicides as shown inU.S. Pat. Nos. 4,335,260; 4,338,473; 4,440,962; 4,501,608; and4,537,992. These known methods of reducing nitrosamine contamination,however, typically involve relatively costly and complicated chemicalprocesses. Accordingly, a relatively simple, effective and inexpensivemethod for reducing nitrosamine impurities in herbicides has long beensought.

Some herbicides are known to exist in two or more polymorphic forms. Forexample, U.S. Pat. No. 4,082,537 discloses thatN-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine [also known asN-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] exists in twodistinct polymorphic forms: a yellow microcrystalline form and an orangemacrocrystalline form. The yellow polymorph, although considered lessstable than the orange polymorph, has the advantage of being slower tosettle out of aqueous dispersions, such as those involved with sprayingequipment. The yellow polymorph is found in freshly preparedN-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine which slowly converts to theorange polymorph with age. However, when sodium dioctyl sulfosuccinateis added to molten N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine followedby cooling, solidification and conversion to a wetable powder, thecompound produces stable aqueous dispersions of the yellow polymorph.The production of herbicidal dispersions which are more stable than theknown herbicidal dispersions is therefore desirable.

SUMMARY OF THE INVENTION

Generally speaking, the present invention relates to herbicidalcompositions in a desired polymorphic form and to a process formicroencapsulating said herbicidal compositions. More particularly, thepresent invention includes a herbicidal composition comprising alpha,alpha, alpha-trifluoro-2,6-dinitro-N,N-di-n-propyl-p-toluidine(trifluralin) having a polymorphic form characterized by a yellow colorand a melting point of approximately 41°-43° C. The present inventionalso includes a method of using the yellow polymorphic form oftrifluralin as a herbicide. In addition, the present invention includesa method of microencapsulating low-melting agriculturally activematerials, such as trifluralin. The microencapsulation process comprisesthe steps of heating a low-melting, water-immiscible, agriculturallyactive material to a molten state. The molten active material is thencombined with an aqueous solution of a water-soluble, film-formingpolymer at a temperature sufficient to maintain the active material inits molten state. The active material is then dispersed or emulsified inthe aqueous solution so that the active material has a primary particlesize of between approximately 0.1 and 10 microns. The resultingdispersion or emulsion is then spray dried at a temperature betweenapproximately 50° and 220° C. so as to microencapsulate the activematerial in the polymer.

In one aspect of the invention, the active material which ismicroencapsulated is the yellow polymorphic form of trifluralin (alpha,alpha, alpha-trifluoro-2,6-dinitro-N,N-di-n-propyl-p-toluidine). Inanother aspect of the invention, the active material which ismicroencapsulated is selected from the group consisting ofN-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzeneamine;3,5-dibromo-4-hydroxybenzonitrile octanoate;4-hydroxy-3,5-diiodobenzonitrile octanoate andmethyl-N,N-dimethyl-N-[(methylcarbamoyl)oxy]-1-thiooxamimidate.

In an alternate embodiment, the invention comprises a method ofmicroencapsulating a desired polymorphic form of a high-meltingagriculturally active material. The microencapsulation process comprisesthe steps of grinding a high-melting, water-immiscible active materialto a primary particle size of between approximately 0.1 and 10 microns.The ground active material is then combined with an aqueous solution ofa water-soluble, film-forming polymer such that said active material isdispersed or emulsified in said aqueous solution. The resultingdispersion or emulsion is then spray dried at a temperature betweenabout 50° and 220° C. so as to microencapsulate the active material inthe polymer. In one aspect of the invention, the active material istetrachloroisophthalonitrile (chlorothalonil). In another aspect of theinvention, the active material is2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine).

Another embodiment of the invention comprises a method of reducingnitrosamine impurities in herbicides. The method comprises the steps ofdispersing or emulsifying a water-immiscible, nitrosamine-containingherbicide in an aqueous solution of a film-forming polymer such thatsaid dispersed or emulsified herbicide has a particle size betweenapproximately 0.1 and 10 microns. The resulting dispersion or emulsionis then spray dried at a temperature between about 50° and 220° C. so asto microencapsulate the herbicide in the polymer, whereby the amount ofnitrosamine in said microencapsulated herbicide is reduced. In oneaspect of the invention, the active material is trifluralin.

Another embodiment of the invention comprises a method for producing adesired polymorphic form of active materials which exhibit two or moreheat-alterable polymorphic forms. The method comprises the steps ofheating to a molten state a water-immiscible active material whichexhibits two or more heat-alterable polymorphic forms, the activematerial being in one polymorphic state prior to heating. The moltenactive material is then combined with an aqueous solution of awater-soluble, film-forming polymer at a temperature sufficient tomaintain the active material in the molten state. The active material isthen dispersed or emulsified in the aqueous solution so that the activematerial has a primary particle size of between approximately 0.1 and 10microns. The resulting dispersion or emulsion is then spray dried so asto microencapsulate the active material in the polymer. Themicroencapsulated active material is then cooled to a temperature belowthe melting point of the active material, whereby the microencapsulatedactive material is in another polymorphic form.

Accordingly, it is an object of the present invention to provideimproved agriculturally active compositions.

Another object of the present invention is to provide a method of usingas herbicides the desired polymorphic form of herbicides, particularlytrifluralin.

A further object of the present invention is to provide an improvedmethod of microencapsulating low-melting agriculturally activematerials.

An additional object of the present invention is to provide an improvedmethod of microencapsulating trifluralin.

Yet another object of the present invention is to provide herbicideshaving improved stability.

Another object of the present invention is to provide trifluralin in asolid, free-flowing form.

Still another object of the present invention is to provide polymorphicherbicides in a form having improved biological activity.

Another object of the present invention is to provide microencapsulatedagriculturally active materials which are capable of being cycled from asolid form, through the melting point of the active material and back toa solid form.

A further object of the present invention is to provide an improvedmethod of microencapsulating high-melting agriculturally activematerials.

Another object of the present invention is to provide an improved methodof reducing nitrosamine impurities in herbicides.

An additional object of the present invention is to provide a method ofconverting agriculturally active materials from one polymorphic form toanother and maintaining or preserving the active material in the otherpolymorphic form.

These and other objects, features and advantages of the presentinvention will become apparent after a review of the following detaileddescription of the disclosed embodiments and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is simplified graphic illustration of the dehydration of anemulsion droplet.

FIG. 2 is a simplified graphic illustration of the emulsion droplet ofFIG. 1 after drying showing encapsulated trifluralin.

FIG. 3 is a differential scanning calorimetry trace of the orangepolymorph of trifluralin.

FIG. 4 is a differential scanning calorimetry trace of the yellowpolymorph of trifluralin.

FIG. 5 is a differential scanning calorimetry trace of a mixture of theyellow and orange polymorph of trifluralin.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

The invention relates to microencapsulation of agriculturally activematerials, such as herbicides, insecticides, fungicides, nematicides,miticides and plant growth regulators. The agriculturally activematerials useful in the present invention can generally be grouped intotwo types of compounds: low-melting and high-melting. Low-meltingagriculturally active materials are generally those which have meltingpoints below approximately 130° C.; high-melting agriculturally activematerials are generally those which have melting points aboveapproximately 130° C. These agriculturally active materials are alsosolid under ambient or room temperature conditions (approximately 20°C.).

One disclosed embodiment of the present invention involves a process forthe microencapsulation of low-melting agriculturally active materials.In this low-melting process, the chemical nature of the agriculturallyactive material is generally not critical. With respect to physicalproperties, the agriculturally active material must be substantiallyimmiscible with water and, of course, must be low-melting as definedabove. The agriculturally active material must also be substantiallyinert with respect to the material which forms the microcapsule wall, aswill be described further hereinbelow.

The low-melting microencapsulation process is performed by firstproviding an aqueous solution of a water-soluble, film-forming polymer.The low-melting, agriculturally active material is then heated to atemperature above its melting point so that the normally solid materialbecomes liquid. The molten active material is then combined with theaqueous phase. The temperature of the aqueous phase must be such thatwhen the molten active material is added thereto, the molten activematerial remains molten. Generally speaking, therefore, the temperatureof the aqueous phase must be at or above the melting point of the activematerial. The active material is then dispersed or emulsified in theaqueous phase by vigorous agitation or mixing with appropriate apparatuswell known in the art, such as high shear mixers or homogenizers. Theobjective of this dispersing or emulsifying step is to convert themolten active material into a plurality of tiny liquid dropletsuniformly dispersed in the aqueous phase. Accordingly, the dispersing oremulsifying step should be conducted so as to provide droplets of liquidactive material having a primary particle size of between approximately0.1 to 10 microns; preferably, between approximately 0.8 and 2.0microns.

The resulting dispersion or emulsion of the active material in theaqueous phase is then spray dried using conventional spray dryingequipment which is well known to those skilled in the art. The purposeof spray drying the dispersion or emulsion is to dehydrate it (FIG. 1)and thereby form a capsule wall of the film-forming polymer around thedroplets of active material. In order to accomplish this dehydrationstep, the dispersion or emulsion is spray dried at a temperature betweenapproximately 50° and 220° C. In terms of conventional spray dryingequipment, the spray drying should be conducted using an air inlettemperature of between approximately 110° and 220° C.; preferably,between approximately 180° and 200° C. An air outlet temperature ofbetween approximately 50° and 120° C.; preferably, between approximately90° and 110° C. is used. The spray drying process produces dry,free-flowing product containing the active material. The free-flowingproduct containing the active ingredient formed by the spray dryingprocess will generally have a particle size of between approximately 10and 500 microns. (FIG. 2).

As stated above, the chemical nature of the low-melting, agriculturallyactive material is generally not critical to the low-meltingmicroencapsulation process. Examples of low-melting, agriculturallyactive materials which can be used in the present invention are asfollows: Senoxan, Cycloxydim, Chloropropham, Diclofop-Methyl, Alachlor,Fluchloralin, Bromoxynil Octanoate Ester, Ioxynil Octanoate Ester,Trifluralin, Pendimethalin, Cyometrinil, Ethalfluralin, Benfluralin,Oxyfluorfen, Ethofumesate, Flamprop-Isopropyl, Napropamide, Benazolin(ester), Monolinuron, Flamprop-Methyl, Bifenox, Fenoxaprop-Ethyl,Propham, Oxadiazon, Mecoprop, Propanil, Prometon, Quizalofop-ethyl,Linuron, Isocarbamid, Metobromuron, Chlorbromuron, Dazomet, Neburon,Terbutryn, Chlomethoxynil, Hexazinone, Dichlorprop, MCPA, Prometryn,Carbetamide, Methabenzthiazuron, Desmedipham, Methazole, Terbumeton,Prodiamine, Metribuzin, Quinalphos, Butocarboxim, Permethrin, EPN,Methidathion, Chlorpyrifos, Phosalone, Dimethoate, Methamidophos,Fenpropathrin, Salithion, Fenoxycarb, Azinphos-Ethyl, Cypermethrin,Tetramethrin, Azinphos-Methyl, MTMC, Methomyl, Xylylcarb, Cloethocarb,Trichlorfon, Acephate, Amitraz, MIPC, Propoxur, Aminocarb, Aldicarb,Deltamethrin, Trimethacarb, Dioxacarb, Methiocarb, Bendiocarb,Vamidothion, Oxamyl and Hexythiazox.

The amount of active material which is added to the aqueous phase is notespecially critical but is generally between approximately 5% and 75% byweight; preferably, between approximately 30% and 50% by weight. Theaqueous phase comprises a solution of between approximately 1% and 50%by weight; preferably, between approximately 5% and 20% by weight, of awater-soluble, film-forming polymer. When the foregoing concentrationsof active material and film-forming polymer are used, the activematerial represents between approximately 10% and 90% by weight;preferably between approximately 50% and 90% by weight, of the finishedmicroencapsulated product.

The water-soluble, film-forming polymers which can be used in connectionwith the present invention are not especially critical. Any suitablewater-soluble, film-forming polymer can be used. Examples ofwater-soluble, film-forming polymers which can be used are: polyvinylalcohol, polyvinylpyrrolidone, starches, modified starches, alginates,hydroxyalkylcellulose, hydroxyalkylcellulose derivatives, poly (acrylicacid), and homologs and salts thereof, polyacrylamide, natural gums,such as gum arabic, dextrins and proteins, such as gelatin and casein. Aparticularly useful water-soluble, film-forming polymer is polyvinylalcohol having a degree of hydrolysis of between approximately 75% and99%; preferably, between approximately 85% and 90%, and a molecularweight of between approximately 10,000 and 100,000; preferably, betweenapproximately 10,000 and 30,000.

Additives to modify the nature of the microcapsule polymer wall can alsobe added to the aqueous phase. Examples of such additives areplasticizers, wetting agents and anticaking agents.

As a part of the present invention, it has been discovered that certainagriculturally active materials exist in more than one polymorphic formone of which has preferred properties. These active materials generallyexist in one polymorphic form in their natural state. This natural stateis usually the more stable polymorphic form. Some of these activematerials can be changed from the more stable polymorphic form toanother less stable polymorphic form by heating the active material andthen cooling to produce the desired form. Generally, heating the activematerial to a temperature above its melting point will convert at leasta portion of the active material from the stable polymorphic form to aless stable polymorphic form upon resolidification. A higher degree ofconversion is obtained, however, if the active material is properlycooled from its molten form. In addition, it has been discovered that anadditional heat treatment slightly above the melting point of the activematerial produces, on solidification, a higher degree of conversion tothe less stable polymorphic form.

Agriculturally active materials which have been discovered to exist intwo or more polymorphic forms include: alpha, alpha,alphatrifluoro-2,6-dinitro-N,N-di-n-propyl-p-toluidine;N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzeneamine;tetrachloroisophthalonitrile,2-chloro-4-ethylamino-6-isopropylamino-S-triazine,4-hydroxy-3,5-diiodobenzonitrile octanoate; andmethyl-N,N-dimethyl-N-[(methylcarbamoyl)oxy]-1-thiooxamimidate. Of theforegoing active materials, all are low-melting, water-immiscibleagriculturally active materials except tetrachloroisophthalonitrile and2-chloro-4-ethylamino-6-isopropylamino-S-triazine which arehigh-melting. The following table lists the above-referenced activematerials and the color and melting points of both their polymorphicforms.

                  TABLE 1                                                         ______________________________________                                                                Less Stable                                                        Stable Polymorph                                                                         Polymorph                                                                    Melting        Melting                                                        Point          Point                                   Active Material                                                                              Color   (°C.)                                                                           Color (°C.)                            ______________________________________                                        alpha, alpha, alpha-                                                                         orange  46°-49°                                                                  yellow                                                                              41°-43°                   trifluoro-2,6-dinitro-                                                        N,N-di-n-propyl-p-                                                            toluidine                                                                     N-(1-ethylpropyl)-                                                                           orange  56°-59°                                                                  yellow                                                                              52°-55°                   3,4-dimethyl-2,6-                                                             dinitrobenzenamine;                                                           tetrachloroisophthalo-                                                                       white   250°-251°                                                                white 247°-248°                 nitrile,                                                                      4-hydroxy-3,5- tan     57°-59°                                                                  tan   53°-56°                   diiodobenzonitrile                                                            octanoate;                                                                    methyl-N,N-    white   108°-110°                                                                white 100°-102°                 dimethyl-N-                                                                   [(methylcarbamoyl)oxy]-                                                       1-thiooxamimidate                                                             2-chloro-4-ethylamino-                                                                       white   182°-185°                                                                white 174°-178°                 6-isopropylamino-s-                                                           triazine                                                                      ______________________________________                                    

In addition, FIG. 3 shows the melting point of the orange polymorphicform of trifluralin; FIG. 4 shows the melting point of the yellowpolymorphic form of trifluralin; and FIG. 5 shows the relative meltingpoints of a mixture of the yellow and orange polymorphic forms oftrifluralin. These traces clearly show the distinctive melting endothermof the two polymorphic forms of trifluralin.

The low-melting microencapsulation process of the present invention isparticularly well suited for use with the above-referencedheat-alterable, polymorphic, agriculturally active materials. Underambient conditions, the less stable polymorphic form tends to revert tothe stable polymorphic form. However, it has been unexpectedlydiscovered that by microencapsulating the active material in the lessstable polymorphic form the active material is preserved and maintainedin the less stable polymorphic form. The microencapsulation processtherefore provides a mechanism by which the less stable polymorphic formof the active material can be rendered storage stable for relativelylong periods of time or at least can be preserved in the less stablepolymorphic form until time of use.

It is also an aspect of the present invention that it has beenunexpectedly discovered that some of the above-referenced polymorphicactive materials, have greater biological activity in the less stablepolymorphic form than in the more stable polymorphic form. For example,the yellow polymorphic form of trifluralin displays greater herbicidalactivity than does the orange polymorphic form of trifluralin. Some ofthe less stable polymorphic forms of the active materials also possessenhanced physical properties over the stable polymorphic form in termsof dispersability in water and stability of aqueous dispersions thereof.For example, the yellow polymorphic form of trifluralin possesses betterproperties of dispersibility in water and storage stability than doesthe orange polymorphic form of trifluralin.

The low-melting, polymorphic, agriculturally active materials areprocessed in the low-melting microencapsulation process of the presentinvention in the same manner as the other low-melting active materialsdescribed above. Initially, the low melting, active material is in itsmost stable polymorphic form. The active material is then heated to amolten state. The molten, active material is then combined with theaqueous solution of the film-forming polymer and the combination isstirred so as to disperse or emulsify the molten active material in theaqueous phase. The resulting emulsion or dispersion is then spray driedso as to microencapsulate the active material. The resulting product isa dry, free-flowing powder or granule.

Following the heating associated with spray drying, themicroencapsulated active material will return to ambient temperatures.At ambient temperatures, the active material within the microcapsulewill return to the solid phase. Generally, the resolidification of theactive material following the melting and the spray drying of the activematerial converts the active material from the stable polymorph to theless stable polymorph. Therefore, the result of the microencapsulationprocess is the microencapsulation of the less stable polymorphic form ofthe active material.

By merely permitting the microcapsules to return to ambient or roomtemperature (approximately 20° C.), the conversion of the activematerial from the stable polymorph to the less stable polymorph may notbe a complete conversion. Therefore, in order to increase the percentageof conversion to the less stable polymorph, the microcapsules arerapidly cooled following spray drying to a temperature of betweenapproximately -20° and 30° C.; preferably, between approximately -5° and5° C. Following the cooling step, when the microcapsules are returned toambient temperatures, the degree of conversion of the active material tothe less stable polymorph is improved.

It has further been discovered that the degree of conversion of theactive material from the stable polymorph to the less stable polymorphcan be improved by subjecting the microcapsules to a prolongedrelatively low temperature heat treatment following the cooling step.Accordingly, following the cooling step, the microcapsules can be heatedto a temperature above the melting point of the active material;preferably, to a temperature of between approximately 50° and 60° C.,for a period of time sufficient to improve the conversion to the lessstable polymorph; preferably, between approximately 30 and 90 minutes.The result of this additional heat treatment step is that the degree ofconversion of the active material to the less stable polymorph isimproved.

In addition to the second heat treatment step, it has been discoveredthat the degree of conversion of the polymorphic active material can beincreased by the use of crystallization initiators or stabilizers.Materials which have been found to help promote the formation(crystallization) of or stabilize and maintain the active material inthe less stable polymorphic form include: organic acids and salts, suchas Benzoic Acid, Sodium Benzoate, Salicylic Acid; 3-Hydroxybenzoic Acidand 4-Hydroxybenzoic Acid; dinitroanilines, such asN-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine, 3,5-dinitro-N⁴,N⁴-dipropylsulfanilamide and 2-dipropylamino-3,5-dinitrobenzotrifluroride;and surfactants, such as Polyoxyethylene(2) cetyl ether, Polyoxyethylene(8) stearate, Diethylene glycol monostearate, Polyethylene glycol 400monostearate, Sorbitan monostearate, Sorbitan monooleate, Sorbitantrioleate, Polyoxyethylene (20) sorbitan monolaurate, Polyoxyethylene(20) sorbitan monooleate and Nonylphenol 4 mole ethoxylate. Materialswhich have been found to help maintain or stabilize the active materialin the less stable polymorphic form include: surfactants, such as Sodiumor Calcium dodecylbenzene sulfonate, TetrasodiumN-(1,2-dicarboxyethyl)-N-octadecylsulfosuccinamate, Dioctyl ester ofsodium sulfosuccinic acid, Sodium N-Methyl-N-oleoyl taurate, Sorbitanmonolaurate, Tetramethyl decynediol polyoxyethylene (20) sorbitanmonostearate, polyoxyethylene (20) sorbitan trioleate andDodecylphenol-9 mole ethoxylate.

The crystallization initiators can be added to the aqueous phase priorto dispersion or emulsification of the active material therein.Alternately, the crystallization initiators can be added to the moltenactive material before dispersion or emulsification in the aqueousphase. The amount of the crystallization initiator which is added to theaqueous phase or molten active material is that amount which iseffective to promote the formation of the desired polymorph, generallybetween approximately 0.1% and 10% by weight; preferably, betweenapproximately 0.5% and 2% by weight.

With the use of the crystallization initiators of the present invention,it has been unexpectedly discovered that the less stable polymorphicform of the active material after it has been microencapsulated can berepeatedly temperature cycled through the active materials' meltingpoint and upon cooling to ambient or room temperature will return to thesolid, less stable polymorphic form. Such conditions of temperaturecycling are often found under typical storage conditions. Therefore, theuse of the crystallization initiators of the present invention makes themicroencapsulated, less stable, polymorphic, active material morestorage stable.

The low-melting active materials which are useful in the presentinvention include dinitroaniline herbicides. It is well known that thetypical manufacturing process for dinitroanilines produces nitrosaminesas a side reaction product. Nitrosamines, of course, are recognizedcarcinogens. Current government regulations place strict limits on thepermissible amounts of nitrosamines which can be present in herbicides.It has been unexpectedly discovered that the use of themicroencapsulation process of the present invention when encapsulatingactive materials which initially contain nitrosamines as an impurityreduces the level of nitrosamines in the microencapsulated product.

Specifically, a dinitroaniline herbicide, such as trifluralin, maycontain as much as 50 parts per million of nitrosamine as an impurityfrom the manufacturing process. Generally, heating dinitroanilineherbicides increases nitrosamine content. Surprisingly, however, bymicroencapsulating such trifluralin using the low-meltingmicroencapsulation process of the present invention, the trifluralinwhich has been microencapsulated by said process contains reduced levelsof nitrosamine contamination compared to the original trifluralinmaterial before being microencapsulated by said process. Although themechanism by which the nitrosamines are removed from the active materialis not understood, it is believed that the removal of water from themicrocapsules during the spray drying process steam distills thenitrosamines from the microcapsule, thereby reducing the level ofnitrosamines remaining in the microcapsule.

The present invention also includes a process for microencapsulatinghigh-melting active materials. In the practice of the high-meltingmicroencapsulation process, the same aqueous solutions of film-formingpolymers are used as can be used with the low-melting microencapsulationprocess described above. However, since the high-melting activematerials are solid at the temperatures involved in the process, it isnecessary to grind the high-melting active material to a relativelysmall particle size. Generally speaking, the high-melting activematerial should be ground to a particle size of between approximately0.1 and 10 microns. The particular equipment used to grind thehigh-melting active material is not critical and several suitable typesof equipment are well known to those skilled in the art.

After the high-melting active material has been ground to an appropriateparticle size, it is combined with the aqueous solution of thefilm-forming polymer. The aqueous solution is agitated during theaddition of the ground active material so that the active material iscompletely and uniformly dispersed within the aqueous solution.Agitation is maintained until the resulting dispersion is spray dried.

The dispersion of the ground, high-melting active material in theaqueous solution of the film-forming polymer is then spray dried in thesame manner as in the low-melting microencapsulation process.

Proper packaging of the spray-dried encapsulated active materialprovides important benefits. In general, any package, such as bottles,jugs, bags, and the like may be used as long as a sufficient moisturebarrier is obtained, to prevent caking of the product due to absorptionof moisture from the air on storage.

For example, the desired polymorphic form of trifluralin may be producedby rapid cooling before or after packaging. If necessary, any undesiredpolymorphic material present in the spray-dried product can be removedby re-melting prior to rapid cooling to a temperature between -20° C.and 30° C. Heating can be accomplished by any suitable means, such asoven or tray heating, fluid bed heating and the like. The product may bepackaged before or after the heating and cooling steps, but finalsealing of the package prior to the cooling step is preferred.

Alternately, a high yield of the desired polymorph can be attained byslow cooling to a temperature between -20° C. and 30° C. In this method,the product can be heated if necessary to eliminate any undesiredpolymorph, but the package should be sealed prior to slow cooling of thespray-dried product. This process results in a high yield of the desiredpolymorph.

Surprisingly, trifluralin in the spray-dried product prepared andpackaged by the above method can be stored at room temperatures aboveits melting point without harming its physical properties or producingthe undesired polymorph.

The microencapsulated active material in accordance with the presentinvention is a dry, free flowing powdery substance. Themicroencapsulated active material can be readily dispersed in water toform a sprayable composition. When the microencapsulated active materialis added to water, the water-soluble, film-forming polymer which formsthe wall of the microcapsule readily dissolves. The result is an aqueousdispersion of the active material. The aqueous dispersion of the activematerial can then be used in a conventional manner, such as by sprayingan area to be treated.

With respect to the less stable polymorphic form of active materials,such as the yellow polymorphic form of trifluralin, the presentinvention provides a means of rendering the less stable polymorphic formrelatively storage stable. Then, when desired, an aqueous dispersion ofthe less stable polymorphic form of the active material can be preparedand applied to an area to be treated. Specifically, in the case of theyellow polymorphic form of trifluralin, an aqueous dispersion of theyellow trifluralin can be prepared and applied to control weeds. It hasbeen unexpectedly discovered that the less stable polymorphic forms ofthe active materials, specifically the yellow polymorphic form oftrifluralin, possess greater biological activity than the more stablepolymorphic form. Furthermore, aqueous dispersions of the less stablepolymorphic active material tend to be more stable than aqueousdispersions of the more stable polymorphic active material.

The following examples are illustrative of the present invention and arenot intended to limit the scope of the invention as set forth in theappended claims. All temperatures are in degrees Celsius and allpercentages are by weight unless otherwise stated.

EXAMPLE 1

157.0 g. of water is heated and maintained at 60° C. To this is slowlyadded 15.7 g. of partially hydrolyzed polyvinyl alcohol (PVA) with adegree of hydrolysis of 87-89% and a molecular weight of 10,000-30,000.The mixture is agitated until the PVA is completely dissolved. To thisaqueous solution is then added 0.1 g. of sodium dioctyl sulfosuccinate.Further stirring is carried out, with the temperature of the solutionbeing maintained at 60° C. 84.2 g. of technical grade trifluralin(a,a,a-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine) is melted byheating to 60° C., is then added to the aqueous solution and emulsifiedwith a high shear homogenizer. The emulsion particle size is furtherreduced by passing the emulsion through a pressure valve homogenizeruntil an average size of 1.1 microns is achieved.

The emulsion is then spray dried at an inlet air temperature of 180° C.,and an outlet air temperature of 104° C. A laboratory type spray dryeris used with two fluid nozzle atomization.

60 g. of a dry free-flowing powder is obtained with a particle size of20-25 microns. The product is then packaged in foil laminate bags. Thismaterial is heated to 55°-60° C. to assure melting of the undesiredpolymorph and then rapidly cooled to 0° C. to effect the solidificationof the trifluralin and produce the desired polymorphic form. Thepresence of the yellow polymorph is confirmed by the use of differentialscanning calorimetry (DSC) which exhibits a distinctive meltingendotherm occurring at 41°-43° C., the melting point of the yellow form.

The product exhibits excellent properties on dilution in water, wettingalmost instantly, producing a stable suspension on standing.

The finished product contains 80% by weight of trifluralin and maintainsits biological and physical properties on storage at both ambient andelevated temperatures.

EXAMPLE 2

157 g. of water is heated to 60° C. and, under moderate agitation 15.7g. of partially hydrolyzed polyvinyl alcohol having a degree ofhydrolysis of 87-89% and a molecular weight of 10,000 to 30,000 is addedthereto. 83.5 g. of technical grade trifluralin is melted, and heated to60° C. 0.84 g. of Toximul T-814 is added to the molten trifluralin, andstirred until a homogeneous mixture is obtained. The molten mixture isadded to the polymer solution and agitated with a high shear homogenizeruntil a uniform dispersion is obtained. A pressure valve emulsifier isthen used to reduce the droplet particle size to 1.0 micron.

The emulsion thus formed is spray dried, using a laboratory spray dryerfitted with a two fluid nozzle atomizer, at an inlet air temperature of180° C., and a outlet air temperature of 106° C. A free-flowing powderis obtained with a particle size of 20-25 microns. The powder is thenpackaged in foil laminate bags.

The powder thus obtained is heated to 55°-60° C. to assure melting ofthe undesired polymorph then rapidly cooled to 0° C. so that thetrifluralin technical solidifies as the yellow polymorph. The producthas excellent properties on dilution with water, wetting rapidly toproduce a stable suspension on standing. It maintains good physical andchemical properties on storage at both ambient and elevatedtemperatures.

EXAMPLE 3

255 g. of water is heated and maintained at 65° C., and to this isslowly added 25.5 g. partially hydrolyzed polyvinyl alcohol with adegree of hydrolysis of 87%-89% and a molecular weight of 10,000-30,000.The mixture is agitated until the PVA is completely dissolved. Furtherstirring is carried out with the temperature being maintained at 65° C.

74.5 g. of technical grade ioxynil (4-hydroxy-3,5-diiodobenzonitrile) asthe octanoate ester is melted by heating to 65° C., is added to theaqueous solution and is emulsified with a high shear homogenizer. Theemulsion particle size is further reduced by passing the emulsionthrough a pressure valve homogenizer until an average particle size of1.2 microns is achieved.

The emulsion is then spray dried at an inlet air temperature of 180° C.and an outlet air temperature of 106° C. A laboratory type spray dryeris used with two-fluid nozzle atomization.

50 g. of a dry free-flowing powder is obtained with a particle size of20-25 microns. The finished product contains predominantly thelow-melting polymorph (melting point 52° C.) with only a few percent ofthe higher-melting polymorph (melting point 57° C.).

The product exhibits excellent properties on dilution in water, wettingalmost instantly, producing a stable suspension on standing.

The finished product contains 70% by weight of ioxynil octanoate andmaintains its physical properties on storage.

EXAMPLE 4

232 g. of water is heated to 60° C. and, under moderate agitation, 23.2g. of partially hydrolyzed polyvinyl alcohol, having a degree ofhydrolysis of 87-89% and a molecular weight of 10,000-30,000 is added.76.8 g. of technical grade bromoxynil(3,5-dibromo-4-hydroxybenzonitrile) as the octanoate ester is melted byheating to 60° C. The molten technical material is added to the polymersolution and agitated with a high shear homogenizer until a uniformdispersion is obtained. A pressure valve homogenizer is then used toreduce the droplet particle size to 1.0 micron.

The emulsion thus formed is spray dried, using a laboratory spray dryerfitted with a two fluid nozzle atomizer, an inlet air temperature of180° C. and an outlet air temperature of 106° C. A free-flowing powderis obtained with a particle size of 10-25 microns.

The product melts at 41.7° C. as determined by DSC. The finished productcontains 70% by weight of bromoxynil octanoate.

EXAMPLE 5

184 g. of water is heated to 60° C. and, under moderate agitation, 18.4g. of partially hydrolyzed polyvinyl alcohol, having a degree ofhydrolysis of 87%-89% and a molecular weight of 10,000-30,000 is added.31.6 g. of technical grade pendimethalin[N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] is melted byheating to 60° C. The molten technical material is added to the polymersolution and agitated with a high shear homogenizer until a uniformdispersion is obtained. A pressure valve homogenizer is then used toreduce the droplet particle size to 1.3 microns.

The emulsion thus formed is spray dried, using a laboratory spray dryerfitted with a two-fluid nozzle atomizer, at an inlet air temperature of180° C., and an outlet air temperature of 106° C. A free-flowing powderis obtained with a particle size of 20-25 microns.

The finished product contains 60% by weight of pendimethalin andmaintains its physical properties on storage at both ambient andelevated temperatures. The finished product contains only the polymorphmelting at 53.2° C., as determined by DSC. The product has excellentproperties on dilution with water, wetting rapidly to produce a stablesuspension on standing.

EXAMPLE 6

314.0 g. of water is heated and maintained at 60° C. To this is slowlyadded 31.4 g. of polyvinylpyrrolidone (PVP) having an average molecularwight of approximately 24,000. The mixture is agitated until the PVP iscompletely dissolved. To this is added 1.68 g. of sodium dodecylbenzenesulfonate. Further stirring is carried out with the temperature beingmaintained at 60° C.

166.92 g. of trifluralin is melted by heating to 60° C. The moltentrifluralin is then added to the aqueous solution and emulsified with ahigh shear homogenizer. The emulsion particle size is further reduced bypassing the emulsion through a pressure valve homogenizer until theaverage particle size of 1.1 microns is achieved.

The emulsion is then spray dried at an inlet air temperature of180°-181° C. and an air outlet temperature of 106°-112° C. A dryfree-flowing powder is obtained with a particle size of 20-25 microns.This product is sealed in a bag and placed in a hot air oven at 55° C.for a period of 1 hour. The polyethylene bag is then foil packed andleft at room temperature for 10 days. DSC analysis reveals that themicrocapsules contain 87.2% of the yellow trifluralin polymorph and nomeasurable percentage of the orange polymorph.

EXAMPLE 7

The same process as described above in Example 6 is followed except thePVP has an average molecular weight of approximately 40,000. DSCanalysis reveals that the microcapsules contain 77.2% of the yellowtrifluralin polymorph and no measurable percentage of the orangepolymorph.

EXAMPLE 8

366.1 g. of water is heated to 60° C. and stirred while 1.95 g. sodiumdodecylbenzene sulfonate and 29.25 g. of 87-89% hydrolyzed polyvinylalcohol (molecular weight 10,000-30,000) are added and dissolved. 195 g.of molten trifluralin are then added with high shear to produce anemulsion. The emulsion particle size is further reduced by passing theemulsion through a pressure valve homogenizer to produce an emulsionparticle size of 1.5 microns.

The emulsion is then spray dried at an inlet air temperature of 188° C.and an outlet air temperature of 107° C., yielding 30 g. of afree-flowing powder with a particle size between 10 and 25 microns. Whencooled from 60° C. to room temperature and held at room temperature for7 days, the desired yellow polymorph is produced in excess of 95%, basedon DSC analysis.

EXAMPLE 9

557.1 g. of water are heated to 60° C. and stirred while 49.4 g. of87-89% hydrolyzed polyvinyl alcohol (molecular weight 10,000-30,000) areslowly added. When the PVA is completely dissolved, 0.3 g. of sodiumdioctyl sulfosuccinate is added.

Separately, 240.0 g. of trifluralin are melted and held at 60° C. withstirring while 0.3 g. of 3-hydroxy benzoic acid is added and dissolved.This solution is then added to the PVA solution and emulsified with ahigh shear homogenizer to produce an emulsion particle size of 1.1microns.

The emulsion is then spray dried in a laboratory type spray dryer at anair inlet temperature of 187° C. and air outlet temperature of 105° C.using two-fluid nozzle atomization. 49.3 g. of a free flowing powder areproduced with a particle size of 15-30 microns.

EXAMPLE 10

The biological activity of the yellow polymorphic form of trifluralinhaving a melting point of approximately 41°-43° C. is compared with thebiological activity of the orange polymorphic form of trifluralin havinga melting point of approximately 46°-49° C. Aqueous dispersions of theorange polymorph, the yellow polymorph and a control are incorporatedwith soil at a rate of 0.5 pounds active ingredient per acre. Thecontrol is made from a commercially available emulsifiable trifluralinconcentrate.

                  TABLE 2                                                         ______________________________________                                        % Control.sup.1 (at 0.5 lbs.a.i. per acre application rate.sup.2                        Barnyardgrass.sup.3                                                                      Crabgrass.sup.4                                                                         Pigweed.sup.5                                  ______________________________________                                        EC.sup.6    39           41        34                                         Orange Polymorph                                                                          53           56        50                                         Yellow Polymorph                                                                          86           82        58                                         ______________________________________                                         .sup.1 % control 14 days after application. 0 = no control, 100 = complet     control                                                                       .sup.2 Application to sandy loam soil                                         .sup.3 Echinochloa crusgalli                                                  .sup.4 Digitaria sanguinalis                                                  .sup.5 Amaranthus retroflexus                                                 .sup.6 Commercially available emulsifiable concentrate (Treflan, Elanco) 

EXAMPLE 11

Another aspect of the invention is the unexpected reductions of theunwanted nitrosamine contaminant that is formed during the manufactureof dinitroaniline herbicides, such as trifluralin. Analysis of theactive ingredient before and after spray drying according to theinvention reveals significant reduction of the nitrosamine contaminant,as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                      Nitrosamine Concentration (ppm)                                               in trifluralin                                                                Sample A  Sample B                                              ______________________________________                                        Before Spray Drying                                                                           1.00        0.15                                              After Spray Drying                                                                            <0.05*      <0.05*                                            ______________________________________                                         *Below detection limit                                                   

EXAMPLE 12

366 g. of water and 195 g. of technical grade chlorothalonil(tetrachloroisophthalonitrile) are added to a laboratory mill and groundto a median particle size of 2-4 microns, while the temperature of thedispersion is maintained at about 70° C. for about four hours. 1.95 g.of sodium dodecylbenzene sulfonate and 37.1 g. of polyvinyl alcoholhaving a degree of hydrolysis of 87-89 percent and a molecular weight of10,000 to 30,000 are then added to the dispersion and dissolved.

The dispersion is then spray dried at an inlet air temperature of 200°C. and an outlet air temperature of 105° C., using a laboratory-typespray drier with two-fluid nozzle atomization. A free-flowing powderwith a particle size of about 15-35 microns is produced which containsthe higher melting polymorph of chlorothalonil, melting at 251.5° C.This polymorph is known as type I, and is characterized by superiorproperties.

EXAMPLE 13

The procedure of Example 4 is followed except that the followingmaterials having the following melting points are separately substitutedfor the bromoxynil octanoate as the agriculturally active material:

    ______________________________________                                        Active Material    Melting Point                                              ______________________________________                                        I.        Diclofop-methyl                                                                            39°                                             II.       Alachlor     39°                                             III.      Senoxan      36°                                             IV.       Oxyfluorfen  65°                                             V.        Flamprop-methyl                                                                            81°                                             VI.       Cypermethrin 60°                                             VII.      Ethalfluralin                                                                              57°                                             VIII.     Benfluralin  65°                                             IX.       Acephate     82°                                             X.        Methomyl     78°                                             XI.       Imazalil     50°                                             ______________________________________                                    

Heating of the active materials is carried out as in Example 4 exceptthat the temperature is adjusted to a temperature above the meltingpoint of the active material.

Each of the above-referenced active materials is microencapsulated inthe polyvinyl alcohol by spray drying. The resulting products are dry,free-flowing powders or granules of the encapsulated active materialslisted above.

EXAMPLE 14

The procedure of Example 12 is followed except that the followingmaterials having the following melting points are separately substitutedfor the chlorothalonil as the agriculturally active material:

    ______________________________________                                        Active Material   Melting Point                                               ______________________________________                                        I.       Oxamyl       102°                                             II.      Deltamethrin  98°                                             III.     Metribuzin   125°                                             IV.      Oryzalin     141°                                             V.       Atrazine     174°                                             VI.      Propanil      91°                                             VII.     Quizalofop-ethyl                                                                            91°                                             ______________________________________                                    

The products of the spray drying process are dry free-flowing powders ofthe above-referenced active materials microencapsulated in polyvinylalcohol.

EXAMPLE 15

The procedure of Example 6 is followed except the followingwater-soluble, film forming polymers are separately substituted for thepolyvinylpyrrolidone:

    ______________________________________                                        I.      Sodium Carboxymethylcellulose                                         II.     Gum Acacia                                                            III.    Poly (Acrylic Acid)                                                   IV.     Casein                                                                V.      Hydrolyzed Maltodextrin (5 Dextrose equivalent)                       VI.     Modified starch                                                       VII.    Starch                                                                VIII.   Polyacrylamide                                                        IX.     Hydroxyethylcellulose.                                                ______________________________________                                    

The products of the spray drying process are dry free-flowing powders oftrifluralin microencapsulated in the above-referenced polymers.

EXAMPLE 16

The procedure of Example 9 is followed except that the followingcrystallization initiators are separately substituted for the 3-hydroxybenzoic acid:

I. 3,5-dinitro-N⁴,N⁴ -dipropylsulfanilamide

II. N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine.

The spray drying process produces a free flowing powder.

EXAMPLE 17

The procedure of Example 8 is followed except that the followingcrystallization initiators are separately substituted for the sodiumdodecylbenzene sulfonate:

I. Sodium N-methyl-N-oleyl taurate

II. Polyoxyethylene (20) sorbitan monostearate

III. Polyoxyethylene (2) cetyl ether

IV. Polyoxyethylene (8) stearate

V. Sorbitan monooleate

VI. Calcium dodecylbenzene sulfonate.

The products of the spray drying are dry free flowing powders of thedesired yellow polymorph of trifluralin produced in excess of 95%, basedon DSC analysis.

EXAMPLE 18

The improved physical properties of the yellow polymorphic form oftrifluralin compared to the orange polymorphic form of trifluralin isshown in Table 4 below:

                  TABLE 4                                                         ______________________________________                                        % Yellow                                                                              % Orange  % Super Cool-                                                                             Suspensa-                                                                            Wetting                                  Solid   Solid     ed Liquid   bility Time                                     ______________________________________                                         0      96         4          32     >60 Secs                                 76       6        18          80      <10 Secs.                               ______________________________________                                    

The foregoing data clearly shows that the yellow polymorphic form oftrifluralin possesses improved properties of dispersability in watercompared to the orange polymorphic form of trifluralin.

We claim:
 1. A process for microencapsulating low-melting,agriculturally active materials which exhibit a plurality ofheat-alterable polymorphic forms comprising the steps of:heating to amolten state a low-melting, water immiscible, agriculturally activematerial which is normally solid at ambient temperatures and exhibits aplurality of heat-alterable polymorphic forms, said active materialbeing in one polymorphic form prior to heating: combining said moltenactive material substantially free of an organic solvent therefor, anaqueous solution of a water-soluble, film-forming polymer and aneffective amount of a crystallization initiating compound selected fromthe group consisting of benzoic acid, sodium benzoate, salicylic acid,3-hydroxybenzoic acid, 4-hydroxybenzoic acid and mixtures thereof at atemperature sufficient to maintain said active material in said moltenstate; dispersing or emulsifying said active material in said aqueoussolution so that said active material has a primary particle size ofbetween approximately 0.1 and 10 microns; spray drying the resultingdispersion or emulsion at a temperature between about 50° and 220° C. soas to microencapsulate said active material in said polymer; and coolingthe resulting microencapsulated active material to substantiallysolidify said active material, whereby at least a portion of theagriculturally active material exhibiting one of the plurality ofpolymorphic forms is converted to another heat-alterable polymorphicform.
 2. A process for microencapsulating low-melting, agriculturallyactive materials which exhibit a plurality of heat-alterable polymorphicforms comprising the steps of:heating to a molten state a low-melting,water immiscible, agriculturally active material which is normally solidat ambient temperatures and exhibits a plurality of heat-alterablepolymorphic forms, said active material being in one polymorphic formprior to heating; combining said molten active material substantiallyfree of an organic solvent therefor, an aqueous solution of awater-soluble, film-forming polymer and an effective amount of acompound selected from the group consisting ofN-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine; 3,5-dinitro-N⁴,N⁴-dipropylsulfanilamide; 2-dipropylamino-3,5-dinitrobenzotrifluoride andmixtures thereof at a temperature sufficient to maintain said activematerial in said molten state; dispersing or emulsifying said activematerial in said aqueous solution so that said active material has aprimary particle size of between approximately 0.1 and 10 microns; spraydrying the resulting dispersion or emulsion at a temperature betweenabout 50° and 220° C. so as to microencapsulate said active material insaid polymer; and cooling the resulting microencapsulated activematerial to substantially solidify said active material, whereby atleast a portion of the agriculturally active material exhibiting one ofthe plurality of polymorphic forms is converted to anotherheat-alterable polymorphic form.
 3. The process of claim 1, furthercomprising the step of cooling said microencapsulated active material toa temperature below the melting point of said active material, wherebythe degree of conversion of the agriculturally active material from theone of said polymorphic forms to the other of said polymorphic forms isimproved.
 4. The process of claim 2, further comprising the step ofcooling said microencapsulated active material to a temperature belowthe melting point of said active material, whereby the degree ofconversion of the agriculturally active material from the one of saidpolymorphic forms to the other of said polymorphic forms is improved. 5.The process of claim 1, wherein said microencapsulated active materialis cooled to a temperature between approximately -20° and 30° C.
 6. Theprocess of claim 2, wherein said microencapsulated active material iscooled to a temperature between approximately -20° and 30° C.
 7. Theprocess of claim 1, wherein said microencapsulated active material iscooled to a temperature between approximately -5° and 5° C.
 8. Theprocess of claim 2, wherein said microencapsulated active material iscooled to a temperature between approximately -5° and 5° C.
 9. Theprocess of claim 1, wherein said low-melting active material has amelting point between approximately 30° and 130° C.
 10. The process ofclaim 2, wherein said low-melting active material has a melting pointbetween approximately 30° and 130° C.
 11. The process of claim 1,wherein said aqueous solution or dispersion is at a temperature ofbetween approximately 40° and 130° C.
 12. The process of claim 2,wherein said aqueous solution or dispersion is at a temperature ofbetween approximately 40° and 130° C.
 13. The process of claim 1,wherein said primary particle size of said active material is betweenapproximately 0.8 and 2.0 microns.
 14. The process of claim 2, whereinsaid primary particle size of said active material is betweenapproximately 0.8 and 2.0 microns.
 15. The process of claim 1, whereinsaid spray drying takes place at an air inlet temperature of betweenapproximately 110° and 220° C.
 16. The process of claim 2, wherein saidspray drying takes place at an air inlet temperature of betweenapproximately 110° and 220° C.
 17. The process of claim 15, wherein saidair inlet temperature is between approximately 180° and 200° C.
 18. Theprocess of claim 16, wherein said air inlet temperature is betweenapproximately 180° and 200° C.
 19. The process of claim 1, furthercomprising the step of combining with said aqueous solution or saidmolten active material, an effective amount of a compound selected fromthe group consisting of sodium dodecylbenzene sulfonate, tetrasodiumN-(1,2-dicarboxyethyl)-N-octadecylsulfosuccinamate, dioctyl ester ofsodium sulfosuccinic acid, sodium N-methyl-N-oleoyl taurate, sorbitanmonolaurate, tetramethyl decynediol, dodecylphenol-9 mole ethoxylate andmixtures thereof.
 20. The process of claim 2, further comprising thestep of combining with said aqueous solution or said molten activematerial, an effective amount of a compound selected from the groupconsisting of sodium dodecylbenzene sulfonate, tetrasodiumN-(1,2-dicarboxyethyl)-N-octadecylsulfosuccinamate, dioctyl ester ofsodium sulfosuccinic acid, sodium N-methyl-N-oleoyl taurate, sorbitanmonolaurate, tetramethyl decynediol, dodecylphenol-9 mole ethoxylate andmixtures thereof.
 21. The process of claim 1, further comprising thestep of combining with said aqueous solution or said molten activematerial, an effective amount of a compound selected from the groupconsisting of tetramethyl decynediol, polyoxyethylene (20) sorbitanmonostearate and polyoxyethylene (20) sorbitan trioleate.
 22. Theprocess of claim 2, further comprising the step of combining with saidaqueous solution or said molten active material, an effective amount ofa compound selected from the group consisting of tetramethyl decynediol,polyoxyethylene (20) sorbitan monostearate and polyoxyethylene (20)sorbitan trioleate.
 23. The process of claim 1, further comprising thestep of combining with said aqueous solution or said molten activematerial, an effective amount of a compound selected from the groupconsisting of polyoxyethylene (2) cetyl ether, polyoxyethylene (8)stearate, diethylene glycol monostearate, polyethylene glycol 400monostearate, sorbitan monostearate, sorbitan monooleate, sorbitantrioleate, polyoxyethylene (20) sorbitan monolaurate, polyoxyethylene(20) sorbitan monooleate, nonylphenol 4 mole ethyoxylate and mixturesthereof.
 24. The process of claim 2, further comprising the step ofcombining with said aqueous solution or said molten active material, aneffective amount of a compound selected from the group consisting ofpolyoxyethylene (2) cetyl ether, polyoxyethylene (8) stearate,diethylene glycol monostearate, polyethylene glycol 400 monostearate,sorbitan monostearate, sorbitan monooleate, sorbitan trioleate,polyoxyethylene (20) sorbitan monolaurate, polyoxyethylene (20) sorbitanmonooleate, nonylphenol 4 mole ethoxylate and mixtures thereof.
 25. Theprocess of claim 1, further comprising the step of heating saidmicroencapsulated active material to a temperature between approximately50° and 60° C. for a period of time between approximately 30 minutes and90 minutes and cooling to a temperature of between -20° and 30° C.,whereby the degree of conversion of the agriculturally active materialfrom the one of said plurality of polymorphic forms to the other of saidplurality of polymorphic forms is improved.
 26. The process of claim 2,further comprising the step of heating said microencapsulated activematerial to a temperature between approximately 50° and 60° C. for aperiod of time between approximately 30 minutes and 90 minutes andcooling to a temperature of between -20° and 30° C., whereby the degreeof conversion of the agriculturally active material from the one of saidplurality of polymorphic forms to the other of said plurality ofpolymorphic forms is improved.
 27. The process of claim 1, wherein saidpolymer is selected from the group consisting of polyvinyl alcohol,polyvinylpyrrolidone, starches, modified starches, alginates,hydroxyalkylcellulose, hydroxyalkylcellulose derivatives, poly (acrylicacid), polyacrylamide, natural gums, dextrins and proteins.
 28. Theprocess of claim 2, wherein said polymer is selected from the groupconsisting of polyvinyl alcohol, polyvinylpyrrolidone, starches,modified starches, alginates, hydroxyalkylcellulose,hydroxyalkylcellulose derivatives, poly (acrylic acid), polyacrylamide,natural gums, dextrins and proteins.
 29. The process of claim 1, whereinsaid polymer is selected from the group consisting of polyvinyl alcohol,polyvinylpyrrolidone, hydroxyalkylcellulose, gum arabic, gelatin andcasein.
 30. The process of claim 2, wherein said polymer is selectedfrom the group consisting of polyvinyl alcohol, polyvinylpyrrolidone,hydroxyalkylcellulose, gum arabic, gelatin and casein.
 31. The processof claim 1, wherein said polymer is polyvinylpyrrolidone.
 32. Theprocess of claim 2, wherein said polymer is polyvinylpyrrolidone. 33.The process of claim 1, wherein said polymer is polyvinyl alcohol. 34.The process of claim 2, wherein said polymer is polyvinyl alcohol. 35.The process of claim 1, wherein said aqueous solution of awater-soluble, film-forming polymer is an aqueous solution of betweenapproximately 1% and 50% by weight of a partially hydrolyzed polyvinylacetate, said polyvinyl acetate having a degree of hydrolysis of betweenapproximately 75% and 99% and a molecular weight of betweenapproximately 10,000 and 100,000.
 36. The process of claim 2, whereinsaid aqueous solution of a water-soluble, film-forming polymer is anaqueous solution of between approximately 1% and 50% by weight of apartially hydrolyzed polyvinyl acetate, said polyvinyl acetate having adegree of hydrolysis of between approximately 75% and 99% and amolecular weight of between approximately 10,000 and 100,000.
 37. Theprocess of claim 1, wherein said active material is trifluralin.
 38. Theprocess of claim 2, wherein said active material is trifluralin.
 39. Theprocess of claim 1, wherein said active material isN-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine.
 40. The processof claim 2, wherein said active material isN-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine.
 41. The processof claim 1, wherein said active material is4-hydroxy-3,5-diiodobenzonitrile octanoate.
 42. The process of claim 2,wherein said active material is 4-hydroxy-3,5-diiodobenzonitrileoctanoate.
 43. The process of claim 1, wherein said active material ismethyl-N,N-dimethyl-N-[(methylcarbamoyl)oxy]-1-thiooxamimidate.
 44. Theprocess of claim 2, wherein said active material ismethyl-N,N-dimethyl-N-[(methylcarbamoyl)oxy]-1-thiooxamimidate.
 45. Aprocess for microencapsulating a herbicidal composition exhibiting aplurality of heat-alterable polymorphic forms comprising the stepsof:heating a herbicidal agent comprising alpha, alpha,alpha-trifluoro-2,6-dinitro-N,N-di-n-propyl-p-toluidine to a temperaturesufficient to melt the herbicidal agent, said herbicidal agentexhibiting one polymorphic form characterized by an orange color andanother polymorphic form characterized by a yellow color, saidherbicidal agent being in said orange polymorphic form prior to heating;combining said heated herbicidal agent substantially free of an organicsolvent therefor, an aqueous solution of between approximately 1% and50% by weight of a partially hydrolyzed polyvinyl acetate, saidpolyvinyl acetate having a degree of hydrolysis of between approximately75% and 99% and a molecular weight of between approximately 10,000 and100,000 and an effective amount of a crystallization initiating compoundselected from the group consisting of benzoic acid, sodium benzoate,salicylic acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid andmixtures thereof at a temperature between approximately 50° and 100° C.;emulsifying said herbicidal agent in said solution so that saidherbicidal composition has a primary particle size of betweenapproximately 0.1 and 10 microns; spray drying said emulsion at atemperature between approximately 50° and 220° C. so as tomicroencapsulate said herbicidal agent in microcapsules of polyvinylalcohol; and cooling the resulting microencapsulated herbicidal agent tosubstantially solidify said herbicidal agent, whereby at least a portionof said herbicidal agent in said microcapsules is converted from saidorange polymorphic form to said yellow form.
 46. A process formicroencapsulating a herbicidal composition exhibiting a plurality ofheat-alterable polymorphic forms comprising the steps of:heating aherbicidal agent comprising alpha, alpha,alpha-trifluoro-2,6-dinitro-N,N-di-n-propyl-p-toluidine to a temperaturesufficient to melt the herbicidal agent, said herbicidal agentexhibiting one polymorphic form characterized by an orange color andanother polymorphic form characterized by a yellow color, saidherbicidal agent being in said orange polymorphic form prior to heating;combining said heated herbicidal agent substantially free of an organicsolvent therefor, an aqueous solution of between approximately 1% and50% by weight of a partially hydrolyzed polyvinyl acetate, saidpolyvinyl acetate having a degree of hydrolysis of between approximately75% and 99% and a molecular weight of between approximately 10,000 and100,000 and an effective amount of a compound selected from the groupconsisting of N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine;3,5-dinitro-N⁴,N⁴ -dipropylsulfanilamide;2-dipropylamino-3,5-dinitrobenzotrifluoride and mixtures thereof at atemperature between approximately 50° and 100° C.; emulsifying saidherbicidal agent in said solution so that said herbicidal compositionhas a primary size of between approximately 0.1 and 10 microns. spraydrying said emulsion at a temperature between approximately 50° and 220°C. so as to microencapsulate said herbicidal agent in microcapsules ofpolyvinyl alcohol; and cooling the resulting microencapsulatedherbicidal agent to substantially solidify said herbicidal agent,whereby at least a portion of said herbicidal agent in saidmicrocapsules is converted from said orange polymorphic form to saidyellow form.
 47. The process of claim 45, further comprising the step ofcooling said microcapsules to a temperature between approximately -20°and 30° C.
 48. The process of claim 46, furhter comprising the step ofcooling said microcapsules to a temperature between approximately -20°and 30° C.
 49. The process of claim 45, further comprising the step ofheating and cooling said microcapsules to a temperature and for a periodof time sufficient to substantially convert said herbicidal agent to apolymorphic form characterized by a yellow color and a melting point ofapproximately 41°-43° C.
 50. The process of claim 46, further comprisingthe step of heating and cooling said microcapsules to a temperature andfor a period of time sufficient to substantially convert said herbicidalagent to a polymorphic form characterized by a yellow color and amelting point of approximately 41°-43° C.
 51. The process of claim 45,further comprising the step of heating said microcapsules to atemperature between approximately 50° and 60° C. for a period of timebetween approximately 30 and 90 minutes.
 52. The process of claim 46,further comprising the step of heating said microcapsules to atemperature between approximately 50° and 60° C. for a period of timebetween approximately 30 and 90 minutes.
 53. The process of claim 45,further comprising the step of combining with said aqueous solution orsaid molten active material, an effective amount of a compound selectedfrom the group consisting of sodium dodecylbenzene sulfonate,tetrasodium N-(1,2-dicarboxyethyl)-N-octadecylsulfosuccinamate, dioctylester of sodium sulfosuccinic acid, sodium N-methyl-N-oleoyl taurate,sorbitan monolaurate, tetramethyl decynediol, dodecylphenol-9 moleethoxylate and mixtures thereof.
 54. The process of claim 46, furthercomprising the step of combining with said aqueous solution or saidmolten active material, an effective amount of a compound selected fromthe group consisting of sodium dodecylbenzene sulfonate, tetrasodiumN-(1,2-dicarboxyethyl)-N-octadecylsulfosuccinamate, dioctyl ester ofsodium sulfosuccinic acid, sodium N-methyl-N-oleoyl taurate, sorbitanmonolaurate, tetramethyl decynediol, dodecylphenol-9 mole ethoxylate andmixtures thereof.
 55. The process of claim 45, further comprising thestep of combining with said aqueous solution or said molten activematerial, an effective amount of a compound selected from the groupconsisting of tetramethyl decynediol, polyoxyethylene (20) sorbitanmonostearate, polyoxyethylene (20) sorbitan trioleate and mixturesthereof.
 56. The process of claim 46, further comprising the step ofcombining with said aqueous solution or said molten active material, aneffective amount of a compound selected from the group consisting oftetramethyl decynediol, polyoxyethylene (20) sorbitan monostearate,polyoxyethylene (20) sorbitan trioleate and mixtures thereof.
 57. Theprocess of claim 45, further comprising the step of combining with saidaqueous solution or said molten active material, an effective amount ofa compound selected from the group consisting of polyoxyethylene (2)cetyl ether, polyoxyethylene (8) stearate, diethylene glycolmonostearate, polyethylene glycol 400 monostearate, sorbitanmonostearate, sorbitan monooleate, sorbitan trioleate, polyoxyethylene(20) sorbitan monolaurate, polyoxyethylene (20) sorbitan monooleate,nonylphenol 4 mole ethoxylate and mixtures thereof.
 58. The process ofclaim 46, further comprising the step of combining with said aqueoussolution or said molten active material, an effective amount of acompound selected from the group consisting of polyoxyethylene (2) cetylether, polyoxyethylene (8) stearate, diethylene glycol monostearate,polyethylene glycol 400 monostearate, sorbitan monostearate, sorbitanmonooleate, sorbitan trioleate, polyoxyethylene (20) sorbitanmonolaurate, polyoxyethylene (20) sorbitan monooleate, nonylphenol 4mole ethoxylate and mixtures thereof.
 59. The process of claim 45,further comprising the steps of packaging said microencapsulatedherbicidal agent while said herbicidal agent is still molten and coolingsaid packaged product to a temperature between approximately -20° and30° C.
 60. The process of claim 46, further comprising the steps ofpackaging said microencapsulated herbicidal agent while said herbicidalagent is still molten and cooling said packaged product to a temperaturebetween approximately -20° and 30° C.
 61. The process of claim 45,further comprising the steps of packaging said microencapsulatedherbicidal agent, heating said packaged product so that said herbicidalagent is remelted and cooling said packaged product to a temperaturebetween approximately -20° and 30° C.
 62. The process of claim 46,further comprising the steps of packaging said microencapsulatedherbicidal agent, heating said packaged product so that said herbicidalagent is remelted and cooling said packaged product to a temperaturebetween approximately -20° and 30° C.